I don’t think anyone knows the answer to that question yet, because Miki’s titan damage is a small sample size at this point. I’ll eat my words and admit I was wrong about Miki (I bashed him before release bc of the slow mana and low health). I have Miki at 3/70 Tarlak at 3/70 and Ranvir +5. While Miki will never see time on a defensive team unless it’s for a tournament he’s a titan hero and a good color stacking option for raids. Here it goes Miki over Tarlak and Ranvir for titans imo of course that’s red, yellow, and blue titans.
With that being said it depends on the titan your alliance is hitting bc Miki can stay alive at 3/70 or with assistance. I’m in the same boat as you I have Ariel +7 with Miki and Frida at 3/70 and my gut is telling me to hold off on giving Frida the scopes for now bc of the titan damage Miki is delivering at 3/70. The choice is between Miki and Frida.
One thing is for sure if I was in your position I’d eventually max them all and have Ariel Aegir Frida and Miki on my red titan team.
I am really struggling between blue ascensions for my defensive team. Would appreciate some help.
Thank you,
Yellow: Guinevere
Purple: Hel
Red: Gravemaker
Green: Alberich
Blue: Athena or Magni
Athena over Magni, no question about it. Athena is one of the best heroes in the game, and useful for everything. 
Thank you for the help!
Athena 100 percent. 20 scopes
Updated after Fenrir’s pull! 
Is Fenrir worth the scopes nowadays or he is very outdated?
My roaster is:
Max - Ariel, Magni, Lord Loki, Frida
3.70 - Aegir, Raffaele, Thorne, Richard, Isarnia
I gave him scopes, as he was my only other blue 5* beside of Lord Loki (already maxed). He is decent, but needs to be paired with a true sniper to shine. I see your Magni is maxed, so I would say that you can’t make a wrong choice by maxing Fenrir. He can be nice in your blue stack/mono.